Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an apparatus and a process for converting an aero gas turbine engine with a fan into an industrial gas turbine engine for electrical power production.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
FIG. 1 shows an aero gas turbine engine with a fan that is used in an aircraft. One such engine is the CFM56 series of aero gas turbine engines. The CFM International CFM56 series is a family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of 18,000 to 34,000 pounds-force (80 to 150 kilonewtons). The CFM56 first ran in 1974 and, despite initial export restrictions, is now one of the most common turbofan aircraft engines in the world, with more than 20,000 having been built in four major variants. It is most widely used on the Boeing 737 airliner and, under military designation F108, replaced the Pratt & Whitney JT3D engines on many KC-135 Stratotankers in the 1980s, creating the KC-135R variant of this aircraft. It is also the only engine (CFM56-5C) used to power the Airbus A340-200 and 300 series. The engine (CFM56-5A and 5B) is also fitted to Airbus A320 series aircraft.
In the aero engine of FIG. 1, a high pressure spool rotates around a low pressure spool. The high pressure spool includes a high pressure compressor (HPC) 11 connected to a high pressure turbine (HPT) 13 through an outer shaft, and a combustor 12 that takes in compressed air from the HPC and produces a hot gas flow from burning a fuel that is passed through the HPT 13. A low pressure turbine (LPT) 15 is located immediately downstream from the HPT 13 and is connected to drive a low pressure compressor (LPC) 14 and a fan 16 through an inner shaft. The LPC 14 compresses air that is passed into the HPC 11.
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
In an industrial gas turbine engine used for electrical power production, during periods of low electrical demand the engine is reduced in power. During periods of low electrical power demand, prior art power plants have a low power mode of 40% to 50% of peak load. At these low power modes, the engine efficiency is very low and thus the cost of electricity is higher than when the engine operates at full speed with the higher efficiency.
Industrial and marine gas turbine engines used today suffer from several major issues that include low component (compressor and turbine) performance for high cycle pressure ratios or low part load component efficiencies or high CO (carbon monoxide) emissions at part load when equipped with low NOx combustors which limit the low power limit at which they are allowed to operate (referred to as the turn-down ratio).
One prior art IGT engine uses a single shaft IGT engine with a compressor connected to a turbine with a direct drive electric generator on the compressor end. Another prior art IGT engine is a dual shaft IGT engine with a high spool shaft and a separate power turbine that directly drives an electric generator. Still another IGT engine is a dual shaft aero derivative gas turbine engine with concentric spools in which a high pressure spool rotates around the low pressure spool, and where a separate low pressure shaft that directly drives an electric generator. Another prior art IGT engine uses a three-shaft IGT engine with a low pressure spool rotating within a high pressure spool, and a separate power turbine that directly drives an electric generator.
The configuration of single shaft IGT engine with a compressor connected to a turbine with a direct drive electric generator is the most common for electric power generation and is limited by non-optimal shaft speeds for achieving high component efficiencies at high pressure ratios. The mass flow inlet and exit capacities are limited structurally by AN2 (last stage blade stress) and tip speeds that limit inlet and exit diameters due to high tip speed induced Mach # losses in the flow. Therefore for a given rotor speed, there is a maximum inlet diameter and corresponding flow capacity for the compressor and exit diameter and flow capacity for the turbine before the compressor and turbine component efficiencies start to drop off due to high Mach # losses.
Since there is a fixed maximum inlet flow at high pressure ratios on a single shaft, the rotor blades start to get very small in the high pressure region of the compressor flow path. The small blade height at a relatively high radius gives high losses due to clearance and leakage affects. High pressure ratio aircraft engines overcome this limitation by introduction of separate high pressure and low pressure shafts. The high pressure shaft (outer spool or shaft) turns at a faster speed allowing for smaller radius while still accomplishing a reasonable work per stage. An example for this is dual shaft aero derivative gas turbine engine with concentric spools, which is typical of an aero-derivative gas turbine engine used for electrical power production. The speed of the high pressure spool is still limited by having a low speed shaft inside the inner diameter (ID) of the high pressure shaft. This drives the high pressure shaft flow path to a higher radius relative to what might otherwise be feasible, which thereby reduces the speed of the high pressure rotor, creating smaller radius blades which reduce the efficiency of the high pressure spool. The dual shaft IGT engine with a high spool shaft and a separate power turbine that directly drives an electric generator arrangement is similarly limited in achieving high component efficiencies at high pressure ratios as the single shaft IGT engine with a compressor connected to a turbine with a direct drive electric generator on the compressor end since the entire compressor is on one shaft.
Turn down ratio is the ratio of the lowest power load at which a gas turbine engine can operate (and still achieve CO emissions below the pollution limit) divided by the full 100% load power. Today's gas turbines have a turn down ratio of around 40%. Some may be able to achieve 30%. Low part load operation requires a combination of low combustor exit temperatures and low inlet mass flows. Low CO emissions require a high enough combustor temperature to complete the combustion process. Since combustion temperature must be maintained to control CO emissions, the best way to reduce power is to reduce the inlet mass flow. Typical single shaft gas turbine engines use multiple stages of compressor with variable guide vanes to reduced inlet mass flow. The limit for the compressor flow reduction is around 50% for single shaft constant rotor speed compressors. The dual shaft aero derivative gas turbine engine arrangement is similarly limited as the single shaft IGT engine arrangement in flow inlet mass flow reduction since the low pressure compressor runs the constant speed of the generator.
Another prior art IGT engine is a three-shaft IGT engine with a low pressure spool rotating within a high pressure spool, and a separate power turbine 4 that directly drives an electric generator, which is the most efficient option of the current configurations for IGT engines, but is not optimal because the low spool shaft rotates within the high spool shaft, and thus a further reduction in the high spool radius cannot be achieved. In addition, if the speed of the low spool shaft is reduced to reduce inlet mass flow, there is a mismatch of angle entering the LPT (Low Pressure Turbine) from the HPT (High Pressure Turbine) and mismatch of the flow angle exiting the LPT and entering the PT (Power Turbine) leading to inefficient turbine performance at part load.